I am new to the Mac world, having just gotten a new MacBook Air for my birthday recently. I was totally freaking out that I would not be able to sync my iTunes library across from Windows XP to the new MacBook. So I'm not sure if my solution will work for you or not, Twentyshots, but it sounds as though your problem is similar in that you are looking for the best way to sync your library to your new device.
We have a friend who is an Apple Genius. He and his wife came over to help me with the transition, because I have no idea what I was doing. He said that because my iTunes library was spread out over several places (mostly on an external drive but some downloads - stuff dl'd for free or purchased at sites other than itunes - were on my laptop's hard drive), the best way to sync everything to one place would be via a home sharing network.
Home sharing will take longer to migrate your data across than if you can transfer everything using an external drive (flash or other hard drive), but our friend said that home sharing will find every song in the iTunes library, regardless of where it is located. He said that I could sync songs downloaded to my other mobile devices too, but I have never downloaded anything to my phone or iTouch, so I didn't have to do that part.
I've got a pretty good sized library and the migration took several days (nonstop) but probably part of that was because our internet speed got hosed after Time Warner took over Insight. And probably because CR was away and I didn't know how to switch off his bit torrenting device, so that was still running and probably slowing down the speed.
Anyway, since you want to sync across several devices, home sharing is probably the way to go, at least based on what my Apple friend recommended.